1. Field of the Invention
The present invention is in the field of spot welding, and more particularly relates to an improved method and apparatus for effecting spot welds between superimposed galvanized metal sheets.
2. The Prior Art
Spot welding has long been recognized as an effective and economical technique for connecting superimposed sheets of metal. In accordance with typical spot welding practice, superimposed metal sheets are clamped between arms defining electrodes. Preferably the electrode tips are pointed and are compressed against registering opposed faces of the sheets.
A high current is applied for a short period of time, resulting in melting of the metal in the area between the tips and fusing together of the sheets in the molten area.
Since welding is effected utilizing low voltages and high amperages, welding can be accomplished only after each of the electrodes is in good electrical contact with the metal. The desired effective electrical contact with the metal is best achieved when the electrodes are sharp and are thus able to penetrate any scale or zinc coatings (in the case of galvanized iron).
The typical metal employed for spot welding electrodes is copper since copper is not compatible with the molten metal formed in the course of the welding operation and thus a copper electrode may be readily lifted clear after a weld has been effected. While copper electrodes have the advantage of incompatibility to the molten material and excellent electrical conductivity, copper electrodes are disadvantageous in that the same are soft, particularly at welding temperatures, and exhibit a tendency to mushroom or flatten at the tip area, with the result that an initially sharp copper electrode which may readily make good contact under forces in the area of 50 to 80 pounds soon becomes rounded or flattened. As a result of such tendency, it is common practice, particularly where spot welds are to be effected between galvanized sheets, for the operator to tilt or wiggle the sheet or the electrodes or both to facilitate at least partial penetration to the base metal. If the current is applied without such penetration, the zinc with which the electrodes are in contact will volatilize, with substantial sparking and spattering and there will be insufficient melting of base metal to effect a proper connection between the sheets.
To compensate for the mushrooming tendencies described and for coating of the electrodes by volatilized zinc, it is common practice for welders to file or redress the points of the electrodes, with resultant reduced productivity and premature consumption of the electrodes.
A further disadvantage of current spot welding techniques, occasioned in large measure by the inability of the welding device to achieve effective contact with the sheets to be connected, is that one or both of the electrodes be water cooled, utilizing an open water cooling system wherein a continuous supply of cold water is circulated, with resultant water wastage. This type of water cooling is required in part because of the excess heat generated by the welding electrodes in burning through the galvanized coating.
It will thus be appreciated that the amount of electricity in the course of forming a spot weld using conventional equipment, and the consequent cooling requirements, could be greatly reduced if good contact with base metal were assured and the energy required to reach base metal eliminated.
A further exposition of the problems of spot welding galvanized metals is contained in U.S. Pat. No. 3,102,190 to Chapleur et al (Aug. 27, 1963) and 3,909,581 to Stone et al (Sept. 30, 1975), which patents propose to facilitate welding respectively by bonding powdered metal to the sheets in the weld area, and by the use of a welding tip alloyed with chromium, beryllium, zirconium, cadmium, silver, gold or platinum.